1. Field of the Invention
The present invention relates to semiconductor wafer cleaning and, more particularly, to techniques for more safely applying a rinsing liquid to the surface of a semiconductor wafer after a fabrication operation.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well-known that there is a need to clean the surface of the wafer after fabrication operations that leave unwanted residues or organic contaminants on the surface of the wafer. Examples of such fabrication operations include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP).
If left on the surface of the wafer for subsequent fabrication operations, unwanted residual material and organic contaminants may cause inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residue and contaminants on the surface of the wafer.
FIG. 1 shows a high-level schematic diagram of a wafer cleaning system 50. The cleaning system 50 typically includes a load station 10 where a plurality of wafers in a cassette 14 may be inserted for cleaning through the system. Once the wafers are inserted into the load station 10, a wafer 12 may be taken from the cassette 14 and moved into a brush box one 16a, where the wafer 12 is scrubbed with selected chemicals and water (e.g., DI water). The wafer 12 is then moved to a brush box two 16b. After the wafer has been scrubbed in the brush boxes 16, the wafer is moved into a spin, rinse, and dry (SRD) station 20 where de-ionized water is sprayed onto the surface of the wafer while the wafer is spun at a speed of between about 100 and 400 revolutions per minute, and then is spun to dry. After the wafer has been placed through the SRD station 20, the wafer is moved to an unload station 22.
By way of example, when the wafer 12 enters the cleaning system 50, the wafer 12 enters the cassette 14 and may be sprayed with water while in the cassette in order to wet the surface. Alternatively, the wafer can be sprayed with water as the wafer 12 enters brush box one 16a or brush box two 16b. Unfortunately, in such water spraying operations, the application of water tends to be non-uniform in that portions of the wafer may receive the application of water before other portions. The portions of the wafer that are sprayed initially will likely undergo unwanted reactions with the chemicals that were on the wafer 12 after the WEB operation. Although the spraying operation may saturate the entire surface of wafer 12, the initial droplets that are applied to the wafer surface will necessarily cause the wafer to have portions of stained surface and portions of non-stained surface. Furthermore, in addition to stains, the technique of spraying water may cause micro-scratches on the surface of the wafer.
Unwanted stains or micro-scratches on the wafer surface may cause, among other things, inappropriate reactions between metallization features. These reactions may destroy the operability of devices on the wafer. A wafer with stains or micro-scratches typically must be discarded, which will ultimately add substantial cost to the overall fabrication process. Unfortunately, the stains or micro-scratches on the surface generally cannot be removed in subsequent cleaning or fabrication operations.
In order to avoid formation of unwanted stains and micro-scratches or introduce other unwanted chemicals onto the wafer surface, some manufacturers will use an additional step by processing the wafer through an SRD station before sending the wafer to the scrubber of brush box one 16a. However, this SRD station will have to be an application specific unit, which will necessarily add significant costs to the construction of a cleaning system 50. Not only does this add significant costs to the station, standard SRD stations are not well configured to perform the pre-cleaning of the wafer surface. The reason standard SRD stations are not well configured is because an SRD station applies a non-controlled spray of water or chemicals over the wafer surface while the wafer is rotated at high speeds. Thus, the incorporation of this additional application specific SRD station will, in most cases, still cause micro-scratching and stains. In view of the foregoing, there is a need for a cleaning process that avoids the problems of the prior art by implementing wafer rinsing techniques for efficiently avoiding the formation of unwanted stains and micro-scratches.
Broadly speaking, the present invention fills these needs by providing a method and system for more safely applying liquid to a surface of a semiconductor wafer. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a system is provided for cleaning a surface of a wafer following a fabrication operation. The system comprises a brush box, which includes a fluid manifold and at least one nozzle. The nozzle is connected to the fluid manifold by a flexible conduit. The nozzle is configured to spray a liquid onto the surface of the wafer at an application angle and at a fan angle. The application angle is defined between a plane of the surface of the wafer and a spraying plane of the liquid. The fan angle and the application angle are configured such that the spraying liquid covers the surface of the wafer in a quiescent manner.
In another embodiment, a method is provided for cleaning a surface of a wafer following a fabrication operation. The method comprises transferring the wafer into a cleaning unit and spraying a fan of liquid onto the surface of the wafer as the wafer is being transferred into the cleaning unit. The fan of liquid is applied at an application angle. The application angle is defined between a plane of the surface of the wafer and a plane of the fan of liquid. The fan of liquid is adjusted to be sprayed onto the surface of the wafer at a fan angle. The fan angle and the application are configured such that the spraying liquid covers the surface of the wafer in a quiescent manner.
In yet another embodiment, a wafer spraying apparatus is provided. The wafer spraying apparatus comprises a manifold for delivering a flow of liquid, a flexible conduit connected to the manifold, and a nozzle. The nozzle is connected to a distal end of the flexible conduit. The flexible conduit is configured to transport the flow of liquid from the manifold to the nozzle. The nozzle is configured to spray a fan of the flow of liquid onto the surface of the wafer. The fan of the flow of liquid is applied at an application angle defined between a plane of the fan of the flow of liquid and a surface of the wafer that is configured to receive the flow of liquid from the nozzle. The nozzle is positioned above the surface of the wafer at a nozzle height. The nozzle height and the application angle are adjustable by way of the flexible conduit.
Advantageously, the present invention provides methods and systems for applying liquid to the surface of a wafer by using a quiescent and symmetrical application technique. As a result, where a wafer has undergone a fabrication operation, the applied liquid will not undergo unwanted reactions with residual chemicals on the wafer surface. In addition to substantially eliminating stains, the techniques of the present invention substantially reduce the number of micro-scratches that may be formed on the wafer surface during cleaning operations. The methods of the present invention are particularly beneficial in post-plasma etching and post-CMP cleaning operations, whereby the wafers are sprayed using the spraying apparatus before commencing normal brush box cleaning.
In addition to protecting from formation of stains and micro-scratches, the spraying technique of the present invention is also suitable for changing a hydrophobic wafer surface into a hydrophilic wafer surface, if desired. Also, the spraying technique eliminates additional application specific mandatory spin, rinse, and dry (SRD) steps prior to brush cleaning in a brush box. Other advantages include improved overall post-CMP and post-WEB clean process flow throughput. Ultimately, the methods and systems disclosed herein will substantially reduce undue costs in the overall fabrication process because the number of damaged wafers that must be discarded will be substantially reduced.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.